This invention relates to rotary mechanical seals of the face type. More particularly it relates to face type mechanical seals utilizing a carbon ring as one of the running sealing faces.
Face type seals are commonly used to provide an essentially fluid tight seal between a housing and a relatively rotating shaft. They find wide application in relatively adverse sealing environments which include high fluid pressures and elevated operating temperatures. Typical applications are water pumps and coolant pumps utilized in nuclear power generation installations. In these latter usages, operating pressures often exceed 2,000 psi. Temperatures of the liquid sealed ranging from 250.degree. F. to 290.degree. F. are not uncommon. Temperatures at the seal itself are maintained at or below 180.degree. F. by circulating quantities of the sealed fluid which have been cooled to the seal area.
Mechanical face seals include two rings pressed together axially, one associated with the stationary housing and one associated with the rotating shaft which are maintained resiliently in sliding sealing contact to prevent leakage of the sealed fluid even after some wear of the sliding faces. It has been well known that use of dissimilar materials to form the sealing rings provides significant advantages in minimization of sliding friction primarily by eliminating spalling of the sliding faces. One common material used for one of the rings is carbon. A variety of other materials may be used to form the cooperating sealing ring. These include steel, ceramic and tungsten carbide.
Use of carbon seal rings in applications which present extremes in terms of fluid pressure has met only limited success. Carbon does not possess sufficient structural rigidity to withstand the differential pressure between the sealed fluid and the exterior of the sealed chamber without considerable distortion. The sealed fluid exerts pressure upon the outer circumferential surface of the carbon seal ring, while the inner annular surface of the carbon ring is exposed to essentially atmospheric pressure. The differential pressure thus produced across the ring causes distortion of the carbon seal ring and its sliding face with a resultant premature failing of the seal.
Use of heavy or large size carbon rings capable of withstanding the high operating pressures has not proven successful. It is difficult to produce a carbon ring of large cross-section which is homogeneous in make-up. These rings have been found to have high internal residual stresses. Both factors contribute to uneven distortion, poor wear characteristics and unsatisfactory seal life.
This invention is directed to elimination of the problems associated with distortion of the carbon sealing ring due to exposure to excessive differential pressure. By this invention the outer circumferential surface of the ring is protected from exposure to the pressure of the sealed fluid. In this way the advantage of utilization of carbon material on one of the sealing faces is rendered available even in extreme pressure applications.